issues with qos and slow network

hi to all
it's the first time i write in this board and actually i'm nearly desperate :)
in the university where i work we've constant network issues...pages that don't load o load just the html without css and an overall slow connection
we have a 2Mbps (yes, two) hdsl line shared among more than 150 users (laptops, desktop, servers, voip..)

the network in structured in this way

isp router --- isp firewall --- zeroshell box --- 3570 switch____ vlan access points
| |_____ vlan classrooms
|_______ vlan offices

the zeroshell box is a linux distribution i use to manage qos / firewalling but can't find a way to make things work
i mean...a slow but working network would be ok, no problem to take 20minutes to buffer a youtube video, but often i've issues even opening the google's home page

do you think that my only option would be upgrading to a faster line...or am i doing something wrong with network configuration?
blocking wireless laptops stop, more or less, the problem but it's not an option...

in qos rules i've classified the subnets giving them max bandwidth and priority, i've also classified http and dns with high priority and dropped in the firewall outgoing udp ports above 1024

any help would be really appreciated!
thanks

Does your Zeroshell box 'know' its only got 2meg to play with or does it think its got its network interface's capacity (100meg?)? Just a thought

I would definitly go for higher bandwidth.. 2mbps is way to slow for 150 users

bio..

Yes, even with QOS with 150 users hitting it at the same time stuff will time out as it would be just lower than 14.4k modem speed (2 megabits / 150 in kilobits - Google Search). Completely unable to handle todays internet even if it was perfectly shared out between them.